As mobile radio communications network technology continues to evolve, a large number of network users employ equipment operating in accordance with Second Generation (2G) specifications in addition, or alternatively, to Third Generation (3G) UMTS specifications. GSM specifications have offered stable functionality, in particular for the Circuit Switched (CS) domain, for many years.
However, with the ongoing development and system evolutions noted above and in accordance with the Third Generation Partnership Protocol (3GPP), for example, for Evolved Packet System (EPS), it has been found that some modifications have become necessary even within the CS domain. In particular, it has become necessary to introduce further information into, for example, initial Non-Access Stratum (NAS) messages such as those arising as part of a location updating procedure, for example the so-called LOCATION UPDATING REQUEST message.
As noted in 3GPP TS 24.008, Release-9 requires the addition of a Circuit Switched fallback Mobile Terminated call (CSMT) flag to be added to the LOCATION UPDATING REQUEST message whereas, in Release-10, a Circuit Switched fallback Mobile Originated call (CSMO) flag was also added.
Thus, it has been found that, for example, the LOCATION UPDATING REQUEST message has been identified as an appropriate host for an ever increasing amount of information and such requirement is likely to continue into the future.
However, a particular issue arises when considering 2G/GSM operation in that the initial NAS messages sent by the UE, such as the LOCATION UPDATING REQUEST message sent when establishing a dedicated connection with the network, exhibits a limitation of size of the message of twenty octets.
This limitation arises from the maximum number of octets in the information field of an Unnumbered Information-Frame (UI-Frame) when sent on the Standalone Dedicated Control Channel SDCCH.
This is clearly evidenced in the following excerpt from 3GPP TS 44.006, “8.8.3 Maximum number of octets in an I, UI, SABM and UA frame partially or entirely available for the information field (N201)
The maximum number of octets partially or entirely available for the information field (N201) is:                for frames of format A and B:                    for the SACCH: N201=18;            for the FACCH and SDCCH: N201=20,                        for frames of format Bbis:                    for BCCH, AGCH, NCH and PCH: N201=23;                        for frames of format Bter:                    for the SACCH: N201=21;            for the FACCH and SDCCH: N201=23;                        for frames of format B4:                    for the SACCH: N201=19.”                        
As noted above, the LOCATION UPDATING REQUEST message is one such message which exhibits a limitation in length which, at present, is as noted in the above-mentioned Technical Specification is twenty octets. The LOCATION
UPDATING REQUEST message has to remain compliant with this maximum length since, as noted in 3GPP TS 44.005, “for unacknowledged operation the data link layer does not offer segmentation services”.
In further detail, and with reference to the 3GPP TS 24.008 sub clause 9.2.15, in Release-8 only two octets are available for extending the LOCATION UPDATING REQUEST message and when employing the IMSI mobile entity. However, and as noted from the above mentioned references to Releases 9 and 10, these two available octets have already found use. While in Release-9, only one octet remained available for extending the LOCATION UPDATING REQUEST, in Release-10, both the spare octets were employed, and no more octets remain available for further extending the content of the LOCATION UPDATING REQUEST message.
While proposals have been made to optimize the usage of any remaining bits within the LOCATION UPDATING REQUEST message, the actual size of the message nevertheless remains a limiting factor.